Independent studio Two Tribes have released their 2D-centric game engine for free via GitHUB under the GNU GPLv2 license.
This game engine has been used across several platforms over the years with some of their best known titles that used the game engine being Toki Tori 2+ and RIVE.
You can view a trailer for their game RIVE which used this engine down below:
In this article:GitHUB, RIVE, Toki Tori 2+, Two Tribes